Peak rate of the Long Term Evolution Advanced (LTE-A) system is much bigger than peak rate of the Long Term Evolution (LTE), required to be 1 Gbps for downlink and 500 Mbps for uplink. Also, the LTE-A system requires good compatibility with the LTE system. Based on the need to raise peak rate, to be compatible with the LTE system and to make full use of spectrum resources, the LTE-A system introduced Carrier Aggregation (CA) technology.
The carrier aggregation technology is: user equipment could work in multiple cells simultaneously, and one cell includes a pair of uplink (UL)/downlink (DL) component carriers (CC). Each component carrier of a carrier aggregation system could be continuous or discrete; the bandwidth of each component carrier could be the same or different; and in order to be compatible with the LTE system, the largest bandwidth of each component carrier is limited to 20 MHz. Carrier aggregation cells of the LTE-A system are divided into primary cells (PCell) and secondary cells (SCell). Only one of the cells aggregated by the user equipments (UE) is defined as the PCell, while all the other cells are defined as SCell.
With rapid development of wireless communications, demands for spectrum resources are growing. Since the mobile data traffic continues to grow, the existing licensed spectrum resources can't fully meet the needs of users, transmission deployment on unlicensed spectrum resources in the LTE system are considered.